Below Zero...

Hot minus temperatures: At a negative absolute temperature the energy distribution of particles inverts in comparison to a positive temperature.

What is normal to most people in winter has so far been impossible in physics: a minus temperature. On the Celsius scale minus temperatures are only surprising in summer. On the absolute temperature scale, which is used by physicists and is also called the Kelvin scale, it is not possible to go below zero—at least not in the sense of getting colder than zero kelvin. According to the physical meaning of temperature, the temperature of a gas is determined by the chaotic movement of its particles—the colder the gas, the slower the particles. At zero kelvin (-273 C) the particles stop moving and all disorder disappears. Thus, nothing can be colder than absolute zero on the Kelvin scale.

Physicists at the Ludwig-Maximilians University Munich and the Max Planck Institute of Quantum Optics in Garching have now created an atomic gas in the laboratory that nonetheless has negative Kelvin values. These negative absolute temperatures have several apparently absurd consequences: Although the atoms in the gas attract each other and give rise to a negative pressure, the gas does not collapse—a behavior that is also postulated for dark energy in cosmology. Supposedly impossible heat engines such as a combustion engine with a thermodynamic efficiency of over 100% can also be realized with the help of negative absolute temperatures.

R and D mag: A temperature below absolute zero